The darkness of Doom 3 makes more sense when you are playing a Thief full conversion

The Dark Mod is a total conversion of Doom 3, but not the Doom 3 BFG Edition apparently, not that you should be to worried with the upcoming open sourcing of the Doom 3 source code. It is not intended to be a remake of the Thief series, the world is similar but the main characters and plot do not follow the original games which will help it feel new even to those who've recently replayed the originals. The actual mod story is fairly short but now there are community made missions available for download to extend your play. Drop by Rock, Paper, SHOTGUN for a link to download the mod and to read the extensive change log of the new patch if you already know about this mod.

"With Dishonored reactivating long-dormant stealth glands the world over, now seems a fine time to revisit perhaps its primary ancestor, the Thief games. Doom 3 total conversion The Dark Mod is a mightily ambitious attempt to recreate Thief – its mechanics if not its actual missions – in a more modern, and very much darkness-orientated, engine. It’s just had a major update and a promising new mission added too."